Error when MGLOfflineStorageDelegate redirects a resource URL to a local file URL

[1ec5]

If you implement -[MGLOfflineStorageDelegate offlineStorage:URLForResourceOfKind:] to redirect a resource URL to a local file URL, such as the URL of a resource bundled with the application, the request fails because the URL response doesn’t come over HTTP:

mapbox-gl-native/platform/darwin/src/http_file_source.mm

Lines 375 to 377 in 9f8c9c6

	// This should never happen.
	response.error = std::make_unique<Error>(Error::Reason::Other,
	"Response class is not NSHTTPURLResponse");

To reproduce this issue, set MGLOfflineStorage.delegate to a class that implements -[MGLOfflineStorageDelegate offlineStorage:URLForResourceOfKind:] like so:

- (NSURL *)offlineStorage:(MGLOfflineStorage *)storage URLForResourceOfKind:(MGLResourceKind)kind withURL:(NSURL *)url {
    if (kind == MGLResourceKindImage && [url.scheme isEqualToString:@"boxmap"]) {
        return [[NSBundle bundleForClass:[self class]] URLForResource:@"glyphs" withExtension:@"png"];
    }
    return url;
}

/cc @mapbox/maps-ios

[1ec5]

#16428 fixes this particular error in a tightly scoped way, which I think is valid. However, I think it’s worth investigating whether local file URLs returned by the delegate should end up going through mbgl::LocalFileSource instead of mbgl::HTTPFileSource, just as they would when loading a style from a file URL.

/cc @mapbox/gl-native